The present invention relates generally to switching devices. More specifically, the present invention relates to improved packaging and circuit integration for electromagnetic devices, such as reed switches and electromagnetic devices such as reed relays.
Electromagnetic relays have been known in the electronics industry for many years. Such electromagnetic relays include the reed relay which incorporates a reed switch. A reed switch is typically a magnetically activated device that typically includes two flat contact tongues which are merged in a hermetically sealed glass tube filled with a protective inert gas or vacuum. The switch is operated by an externally generated magnetic field, either from a coil or a permanent magnet. When the external magnetic field is enabled, the overlapping contact tongue ends attract each other and ultimately come into contact to close the switch. When the magnetic field is removed, the contact tongues demagnetize and spring back to return to their rest positions, thus opening the switch. It is also possible that the switch does not have a glass envelope and is not actuated by magnetic force. For example, the envelope may be made of other materials, such as copper, and can be actuated by other forces, such as centripetal, centrifugal and acceleration forces.
Reed switches, actuated by a magnetic coil, are typically housed within a bobbin or spool-like member. A coil of wire is wrapped about the outside of the bobbin and connected to a source of electric current. The current flowing through the coil creates the desired magnetic field to actuate the reed switch within the bobbin housing.
FIGS. 1-3 shows further details of the configuration of such a prior art reed switch device discussed above. Turning first to FIG. 1, a perspective view of a prior art reed switch configuration 10 is shown. A known reed switch 11 includes, preferably, a glass envelope 12 as well as two signal leads 14 emanating from opposing ends of the reed switch 11 and coil termination leads 15. The signal leads are connected to a pair of metal contacts 13. It should be noted that other envelopes, such as metal, may be used in a switch that is actuated by other forces, such as centripetal, centrifugal and other acceleration forces. The construction of a reed switch 11 is so well known in the art, the details thereof need not be discussed. A shield conductor 16, commonly made of brass or copper, is provided in the form of a cylindrical sleeve which receives and houses the reed switch 11. The reed switch 11 and shield 16 are housed within the central bore 18 of a bobbin or spool 20. About the bobbin 20 is wound a conductive wire 22. As a result, a co-axial arrangement is formed to protect the reed switch 11 device and to control the impedance of the environment and to improve the overall transmission of the signal. The reed switch 11, shield conductor 16 and bobbin 20 are shown in general as cylindrical in configuration. It should be understood that various other configurations, such as those oval in cross-section, may be employed and still be within the scope of the present invention.
As can be understood and known in the prior art, the free ends of the coil of wire 22, the shield 16 and signal terminals 14 of the reed switch 11 are electrically interconnected to a circuit as desired. The respective components of the reed switch 11 configuration are interconnected to a circuit by lead frame or other electrical interconnection (not shown). The lead frame or other electrical interconnection introduces a discontinuity of the desirable co-axial environment.
As described above, the overall reed switch device 10 must be designed to be easily accommodated within a user's circuit. For example, a circuit used to operate at high frequency is designed with a defined characteristic impedance environment. The goal of designing and manufacturing a reed device 10 to the specifications of a circuit customer is to match the desired impedance of the device 10 to the circuit environment as closely as possible. It is preferred that there is no discontinuity of impedance from the reed device 10 itself to a circuit board trace of the circuit that will receive the device 10. The characteristic impedance, Z1, is generally a function of the outer diameter of the signal conductor 14, the inner diameter of the shield 16 and the dielectric constant of the insulation (not shown) between the signal conductor 14 and the shield 16.
A further modification of the reed switch package of FIG. 1 is shown in FIGS. 2-3. A reed switch device 103 is provided to include an outer bobbin 102 with coil 109 wrapped around it for introducing the necessary magnetic field to actuate the reed switch 111. Ends of wire 109 may be connected to posts, pins, or the like (not shown) connected to bobbin 102 to provide for electrical interconnection of the magnetic field current. Emanating from the reed switch 111 are two signal leads 106 which correspond to opposing sides of the reed switch 111. Also emanating from the bobbin body 102 are a pair of shield or ground tabs 108 on each side of the bobbin body 102 that are electrically interconnected to, as shown in FIG. 6, the ends of the inner shield sleeve 110. As shown in FIG. 3, an exploded perspective view the reed switch 111 of FIG. 2, these ground tabs 108 are extensions from the shield sleeve 110 itself on opposing sides thereof.
In particular, the reed switch 111 includes a signal conductor 106 within a glass capsule 126 with an inert gas or vacuum 128 surrounding it. Positioned about the glass capsule 126 is a ground shield 130 which is preferably of a cylindrical or tubular configuration but may be of an oval cross-section to accommodate certain reed switches 111 or multiple reed switches in a multiple channel environment. The foregoing assembly is housed within the bobbin 102 which includes an energizing coil 109.
Some applications of reed devices require the switch to carry signals with frequencies in excess of 500 MHz. However, there is a continuing need for reed relays to transmit higher and higher frequencies without significant attenuation of the transmitted signal power. Current reed relays can operate up to the range of 8-10 GHz.
However, there is even a further need to increase these operating bandwidth ranges to 18 GHz and possibly even higher. In general, there is a need for a reed relay to have very high RF performance where the RF path is optimized to minimize impedance discontinuities throughout the signal path and to reduce stub capacitance.
In the prior art, it is common for individual reed switches to be employed to form various type of switching functions so that they may be incorporated into a circuit, such as a circuit board for automated test equipment (ATE). For example, as in FIG. 4, a reed switch may be employed as a single throw switching device 50 with a single pole 52. This is known as a “Form A” configuration. Also, a Form C switching environment is possible, as shown in FIG. 5 where a single switch 54 can throw to two different poles 56, 58. It can be understood, such multi-pole switching adds complexity to the device with a higher cost. To address this, “pseudo” Form C configurations are commonly employed in the prior art to simplify the switching and to enable the use of individual reed switch devices that are readily available at relative low cost. Such as “pseudo” Form C switching configuration is shown in the switch arrangement 60 seen in FIG. 6. Two Form A switches 62, 64 are used with a bridge 66 to achieve this configuration. As can be understood, with the appropriate connection comprised of the leads of the switches and traces on a circuit board, the appropriate switching capability can be incorporated into a circuit on a circuit board, such as in automated test equipment (ATE).
However, as is well known in the art, this results in a long, unprotected and vulnerable connection between the terminals of the reed switches and the circuit board which is commonly termed a “stub connection.” As a result of this long, unprotected stub connection, significant parasitic capacitance C to ground will be present. This is termed a “stub capacitance” and acts to load the high frequency path, thus limiting the frequency of the circuit to a value in the range of about 5.0 GHz, for example. However, to properly test very fast devices under test (DUT), such as high-speed microprocessors, the frequency of the test circuit must reach the 7 GHz range and even higher, such as 18 GHz and above. Unfortunately, prior art reed switch devices configurations include a stub connection on the circuit board that makes the device essentially incapable of testing high-speed devices.
The foregoing shortcomings in the prior art can be readily understood after viewing an actual circuit into which such a Form C or “pseudo” Form C arrangement of reed switches are incorporated. FIGS. 7 and 8 illustrate such an example circuit environment. Circuit 300 is one that is commonly employed in ATE (Automated Test Equipment) for the purpose of testing circuit devices, generally referenced as 313, and the like. This circuit 300 sets forth a three terminal device that may be “stackable” in series, end to end, depending on the application. A three terminal device 306 with a first reed switch 302 and a second reed switch 304 is shown in FIG. 7 as generally referenced by the dotted lines. For example, the first reed switch device 302 provides a connection for a high frequency AC signal while the second reed switch 304 provides a connection for a DC signal or low frequency AC signal.
More specifically, a signal generator 308 is connected to the first terminal 310 of the first reed switch 302. A second reed switch 304 is provided with a first terminal 312 and a second terminal 314. A second terminal 316 of the first reed switch 302 is connected to the second terminal 314 of the second reed switch 304 at node 318. This node 318 becomes the output terminal 326 to the device 306. A second pair of reed switches 320, 322 is employed to receive the stimulus from the device under test, (DUT) 313. Receiver 317 receives the output from the second pair of reed switches 320, 322. The serial nature of the pair of switches enables a circuit to be designed with a number of different test operations to a different number of DUTs which are independently selectable and isolatable. FIG. 8 illustrates a representational schematic of one of the pair of reed relays that carry out the circuit diagram of FIG. 7.
To carry out this circuit, two individual reed switches are connected to a circuit board (not shown) with the appropriate connection 324 comprised of the leads of the switches and the trace on the circuit board therebetween. This results in a long, unprotected and vulnerable connection between the terminals of the reed switches and the circuit board which is commonly termed a “stub connection.” As a result of this long, unprotected stub connection 324, significant parasitic capacitance C to ground will be present. This is termed a “stub capacitance” and acts to load the high frequency path, thus limiting the frequency of the circuit to a value in the range of about 5.0 GHz, for example. However, to properly test very fast devices under test (DUT), such as high-speed microprocessors, the frequency of the test circuit must reach the 7 GHz range and higher, such as 18 GHz, in the future. Therefore, with a prior art mounting of the reed switches 302, 304 and stub connection 324 on the circuit board, this circuitry 300 is incapable of testing high-speed devices. The protection of a this stub connection is an example of many different ways to employ the present invention.
Another concern in the industry concerns impedance matching of the switch to the circuit into which it is installed. Currently available reed devices are incorporated into a given circuit environment by users. For application at higher frequencies, such as in the 18 GHz range and higher, as is well known in the art, a reed switch is ideally configured to match as closely as possible the desired impedance requirements of the circuit, such as 50 ohms, in which it is installed.
To address these impedance matching needs, within a circuit environment, a co-axial arrangement is preferred throughout the entire environment to maintain circuit integrity and the desired matched impedance. As stated above, the body of a reed switch includes the necessary co-axial environment. In addition, the signal trace on the user's circuit board commonly includes a “grounded co-planar waveguide” where two ground leads reside on opposing sides of the signal lead and in the same plane or a “strip line” where a ground plane resides below the plane of the signal conductor. These techniques properly employed provide a controlled impedance transmission line which is acceptable for maintaining the desired impedance for proper circuit function.
This is due to, for example, the fact that the reed switch itself must be physically packaged and electrically interconnected to a circuit board carrying a given circuit configuration. It is common to terminate the shield and signal terminals to a lead frame architecture and enclose the entire assembly in a dielectric material like plastic for manufacturing and packaging ease. These leads may be formed in a gull-wing or “J” shape for surface mount capability. The signal leads or terminals exit out of the reed switch body and into the air in order to make the electrical interconnection to the circuit board. This transition of the signal leads from plastic dielectric to air creates an undesirable discontinuity of the protective co-axial environment found within the body of the switch itself. Such discontinuity creates inaccuracy and uncertainty in the impedance of the reed switch device.
As a result, circuit designers must compensate for this problem by specifically designing their circuits to accommodate and anticipate the inherent problems associated with the discontinuity of the protective co-axial environment and the degradation of the rated impedance of the reed switch device. For example, the circuit may be tuned to compensate for the discontinuity by adding parasitic inductance and capacitance. This method of discontinuity compensation is not preferred because it complicates and slows the design process and can degrade the integrity of the circuit. This is particularly problematic with very high frequency circuit environments, such as ones that operate in the 18 GHz and higher.
However, such tuning compensation schemes only work over a relatively narrow range of frequency. There is a demand to reduce the need to tune the circuit as described above. The prior art uses a structure of carefully designed vias, which are expensive and difficult to manufacture, to control the impedance from the relay to the board transition.
In view of the foregoing, there is a demand for a reed switch device that can reduce the parasitic stub capacitance to achieve higher frequency signals, such as those in the range of 18 GHz and higher. There is a further need to increase RF performance in such a reed switch device environment. There is also a demand for a reed switch device that includes a controlled impedance environment through the entire body of the package to the interconnection to a circuit. There is a particular demand for a reed switch device to be compact and of a low profile for installation into small spaces and for circuit board stacking. There is further a demand for reed switch devices that are of a surface mount configuration to optimize the high frequency of the performance of the system. Further, there is a demand for a reed switch device that can reduce the need to tune a circuit to compensate for an uncontrolled impedance environment. Also, there is a demand for a reed switch device that has a small footprint and is of a standard shape and configuration for simplified manufacture and installation.
Still further, there is a demand for a reed switch device that is capable of performing much faster than prior art reed switch devices, such as in the 18 GHz range and even higher. There is a need for a reed switch device that is suitable for Form C and Form A applications. There is a need to filter out high frequency in the GHz range for improved operation of the device at very high frequencies, such as those in the 18 GHz range and higher. There is a particular need to reduce the degree of attenuation of high frequency signals. There is a desire to match and interconnect the device to a given circuit, such as one that operates in the 50 ohm range. There is a need to optimize the operation of the circuit into which the reed switch device is installed to simulate a co-axial environment. There is also a need to be able to add DC voltage to the high frequency signal. There is yet another need in the prior art to minimize impedance discontinuities by altering the configuration of the shielding of the device.